Understanding image size in Photoshop
The image size setting is very useful. Understanding its function can be more useful. To access image size got to image in the top menu bar and choose image size. The settings found here can be used to set up the print size of your images and adjust the resolution.
Pixel dimensions: This area lets you set the actual size in pixels of the document. You can set the width and height in pixels or percent. If you are using percentages the sizes are changed relative to the original size.
Document Size: The changes made in this section will not affect the pixel dimensions if the Resample Image box is not checked. If the box is checked the pixel count will change with the changes that you make. This section is used to change the print size of a document. If the resample image box is not checked when you change the dimensions, the resolution will automatically change proportionately. Pay attention to the resolution. 300 pixels/ inch is the quality of magazine prints. If your resolution is below 150 you might start to notice pixelation.
The constrain proportions box will keep the width and height proportional to each other. Keep this box checked to avoid image distortion.